1. Field of the Invention
This invention relates to an address extending control unit, and more particularly to an address extending control unit for a data processing system which is provided with a memory having a memory capacity larger than that assignable with the content of an address register.
2. Description of the Prior Art
In the prior art, a data processing system employing a 16-bit address register is incapable of an address assignment of more than 64K and a usable main memory capacity is limited to 64K. Recently, however, there is a demand for increasing the capacity of the main memory by reason of speeding up of processing and easy programming. On the other hand, in the aspect of software, there is a demand for the employment of common software having no connection with the extending of the main memory capacity, that is, the problem of the so-called software compatibility.
To meet such demands, the method that has been proposed in connection with a data processing unit of, for example, 16 bits, is to extend the main memory capacity without increasing the bit number. Typical examples of this method are a base register method and a bank method. However, the former becomes heavy in terms of both hardware and software and this does not matter in large-sized machines but remains an unsettled problem in connection with medium- and small-sized machines. In the latter, since a physical memory area of the main memory is given in the unit of a bank, this method is not always practical from the viewpoint of actual operation of the data processing system.